It really depends on specifics of the program you want to try to manipulate.

If it is a standard Win32 application, then you could use a program like Spy++ to dig into the forms and get names of controls so you could send Win32 API messages to get values from textboxes and labels and things like that. You can also use this to invoke commands upon the other software. Another method of sending commands is SendKeys, but usually SendKeys never results in a 100% ideal scenario.

There was just an article in MSDN magazine about UI test automation, which may be of some interest for you to read. It isn't in VB (C#), but the concepts are all the same and easily applied to VB.

http://msdn.microsoft.com/msdnmag/issues/08/02/TestRun/